Optical methods of non-destructive quality control are used for examination of surface relief and for flaw detection in a variety of articles. In these inspection methods light reflected from article's surface, or light that has come through a defect that penetrates completely through the test article (penetrating defect) is registered by a photo-detector and used as the information signal. Existing methods for detection of defects that penetrate through foils and films have several disadvantages.
The method detection of defects in a circular or spiral diffraction grating”, a device detecting topographic defects in the process of optic control of round and spiral tracks on article's surface is proposed in literature. These tracks set a diffraction grating analogous to those existing on a video-disk or an optical recording disk. Irregularity of spatial track distribution is registered by characteristics of diffraction reflection from surface, reflection being observed by means of a TV camera or a monitor.
In the literature the apparatus for defect inspection in an object having a periodic pattern is considered. This apparatus includes a system which illuminates the object with parallel light in a direction given by the first pre-set inclination angle relative to the object's surface being inspected. A sensitive cell uses an objective. Information on defects is obtained through analysis of object's image. Mutual positioning between illumination direction and image registration direction is adjusted based on diffraction angle which is determined using image period and wavelength of illumination light. Sum of aperture angle of illumination light and aperture angle of reception objective is set to a lesser value than the diffraction angle.
Other method and apparatus for detection of fine defects with high resolution of an optical image of such defects is also considered in literature. Difference achieved in contrasting image details is greater than fine lines of a semi-conductor device image. Apparatus includes a sample-mounting device, an illuminating apparatus and one detecting the reflected image, a display for optic image registration, device for adjusting parameters of the illuminating and the detecting apparatuses, a device for saving data of the images being compared and a device pointing out defects by comparing the registered and the saved optical image.
The device for detecting defects through control of light reflection off the substrate covered with transparent coating is known. The device includes polychromatic spotlight whose light beam is directed at the observed site. The direction of the light beam makes an angle greater than π−β, where β is the Brewster flat-surface angle. The device also contains a registration apparatus whose optic axis is located in the same plane with the light beam, at least the first secondary source of monochromatic light, a further localized registration apparatus oriented, along with the light source, at the observed site, spotlight carrier, registration apparatus carrier and secondary source carrier, and a device for controlling carrier's relative movement in relation with the controlled surface in at least two directions parallel to this surface.
However all methods and device shortly described abovementioned allow extract information about defects based on analysis of images obtained in the light reflected off the controlled surface. It is characteristic for detection and evaluation of geometric dimensions of non-through surface defects of the articles examined. At times, article's construction itself does not allow one to examine light passing through a transverse defect. These methods and apparatuses do not consider possibility of simultaneous defectoscopy and defectometry of a detected through defect with corresponding change in position of the light-sensitive unit and computer processing of the fixed images in real time.
At the same time, the method and apparatus that are presented in the current patent application and are based on the researches that were conducted using various registration equipment, including microscopes, has shown that in studying through the defects in thin metal foil or opaque film, the measurement methods based on registration of the light passed through a defect are distinguished by a higher sensitivity.